Important note to start with: I am not a lawyer, nor do I play one on TV, but I am a Web Developer, and have developed websites and applications that rely on boilerplate-style Terms of Service agreements like this one, so I have some experience in interpreting what's necessary for a site to run. My words and interpretation do not constitute legal advice and should not be relied on in a court of law.
Congratulations, you're now a little more informed, though given the dense legalese these contracts have probably not as much as you thought, and I'm fairly sure you're now freaking out about some clause that grants some kind of nebulously defined rights and licenses. So let's break down the licensing clauses in most common boilerplate ToS statements, and see what they say and why they say it.
By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license (with the right to sublicense) to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content in any and all media or distribution methods now known or later developed (for clarity, these rights include, for example, curating, transforming, and translating). This license authorizes us to make your Content available to the rest of the world and to let others do the same. You agree that this license includes the right for us to provide, promote, and improve the Services and to make Content submitted to or through the Services available to other companies, organizations or individuals for the syndication, broadcast, distribution, promotion or publication of such Content on other media and services, subject to our terms and conditions for such Content use. Such additional uses by us, or other companies, organizations or individuals, is made with no compensation paid to you with respect to the Content that you submit, post, transmit or otherwise make available through the Services as the use of the Services by you is hereby agreed as being sufficient compensation for the Content and grant of rights herein.
You represent and warrant that you have, or have obtained, all rights, licenses, consents, permissions, power and/or authority necessary to grant the rights granted herein for any Content that you submit, post or display on or through the Services. You agree that such Content will not contain material subject to copyright or other proprietary rights, unless you have necessary permission or are otherwise legally entitled to post the material and to grant us the license described above.
Woof, that's some legal jargon. So let's break it down:
By submitting, posting or displaying Content on or through the Services, you grant us a worldwide, non-exclusive, royalty-free license...
When you post something, you automatically grant them a non-exclusive (so it doesn't restrict what you can do with it, they have no rights to stop you posting it elsewhere or whatever) and royalty-free (that is, they don't have to pay you to use it) license.
...(with the right to sublicense)...
This is a tricky one, but it's there for a reason. We'll get back to it in a minute.
...to use, copy, reproduce, process, adapt, modify, publish, transmit, display and distribute such Content...
These are all ways of dealing with content that are necessary for a website or app to function. For instance, "reproduce", "copy", "transmit", "display" and "distribute" are all methods to get your content to other people. "Adapt" and "modify" are in there because your content might be reformatted (pictures cropped, for instance, or a PNG upload converted to JPG for better compression, or a particularly long message shortened with a link to view the full text) in some way to make it easier to display to other people.
...in any and all media or distribution methods now known or later developed (for clarity, these rights include, for example, curating, transforming, and translating)...
"Curating" would be making a "best-of" compilation, for instance. Transforming and translating would also refer to making content available in different languages than it was originally sent. And "any and all media now known or later developed" - If the Web 3.0 comes around, they still want to be able to send your messages to people in that too.
Now, back to that sublicensing part - When someone quotes your message to reply to it, they need a license to be able to reproduce what you posted. This allows them to grant that license on your behalf.
This license authorizes us to make your Content available to the rest of the world and to let others do the same.
Well, that's relatively simple.
You agree that this license includes the right for us to provide, promote, and improve the Services and to make Content submitted to or through the Services available to other companies, organizations or individuals for the syndication, broadcast, distribution, promotion or publication of such Content on other media and services, subject to our terms and conditions for such Content use.
You grant them the right for them to make changes and make things better, to make ads for the service, and because the site enables embedding (that is having content from users shown on other sites, or shown as part of a news story, or whatever), the company needs the ability to grant those sites a license to display your content, same way you grant them a license to do so. This granted license is also governed by these terms of service.
Such additional uses by us, or other companies, organizations or individuals, is made with no compensation paid to you with respect to the Content that you submit, post, transmit or otherwise make available through the Services as the use of the Services by you is hereby agreed as being sufficient compensation for the Content and grant of rights herein.
If someone else uses your post, you aren't getting paid for it (at least, through here).
And finally:
You represent and warrant that you have, or have obtained, all rights, licenses, consents, permissions, power and/or authority necessary to grant the rights granted herein for any Content that you submit, post or display on or through the Services. You agree that such Content will not contain material subject to copyright or other proprietary rights, unless you have necessary permission or are otherwise legally entitled to post the material and to grant us the license described above.
If you upload something, you are stating you have the legal right to do so, that you own it and/or can grant the licenses mentioned above. So uploading a movie would break this ToS (for example) unless you were the movie studio that made it.
This is standard, boilerplate stuff you will find in every single Terms of Service agreement for any website or app that deals with User-generated content.